APS are solid state imagers where each pixel contains a photosensing means and other active components creating a charge that is converted to a voltage or current. The signal represents the amount of light incident upon a pixel photosite. The dynamic range (DR) of an imaging sensing device is defined as the ratio of the effective maximum detectable signal level, typically referred to as the saturation signal, (V.sub.sat), with respect to the rms noise level of the sensor, (.sigma..sub.noise). This is shown in equation 1. EQU Dynamic Range=V.sub.sat /.sigma..sub.noise Equation 1
Image sensor devices such as charge coupled devices (CCD) that integrate charge created by incident photons have dynamic range limited by the amount of charge that can be collected and held in a given photosite, (V.sub.sat). For example, for any given CCD, the amount of charge that can be collected and detected in a pixel is proportional to the pixel area. Thus for a commercial device used in a megapixel digital still camera (DSC), the number of electrons representing Vsat is on the order of 13,000 to 20,000 electrons. If the incident light is very bright and creates more electrons that can be held in the pixel or photodetector, these excess electrons are extracted by the anti-blooming means in the pixel and do not contribute to an increased saturation signal. Hence, the maximum detectable signal level is limited to the amount of charge that can be held in the photodetector or pixel. The DR is also limited by the sensor noise level, .sigma..sub.noise. Due to the limitations on Vsat, much work has been done in CCD's to decrease .sigma..sub.noise to very low levels. Typical commercial megapixel DSC devices have a DR of 1000:1 or less.
The same limitations on DR exist is for APS devices. The V.sub.sat is limited by the amount of charge that can be held and isolated in the photodetector. Excess charge is lost. This can become even more problematic with APS compared to CCD due to the active components in the APS pixel limiting the area available for the photodetector, and due to the low voltage supply and clocks used in APS devices. In addition, since APS devices have been used to provide image sensor systems on a chip, the digital and analog circuits used on APS devices such as timing and control and analog to digital conversion, that are not present on CCD's, provide a much higher noise floor on APS devices compared to CCD. This is due to higher temporal noise as well as possibly quantization noise from the on-chip analog to digital converter.
U.S. Pat. No. 5,650,643 issued to Konuma (Konuma) teaches a device that can be used to increase the dynamic range of solid state image sensing devices. Konuma shows a means to increase the effective V.sub.sat level by incorporating a comparator and a counter associated with a photodetector to measure the time required to reach an integrated signal threshold level, and provide this as the only sensor output. The counter is used in conjunction with the comparator to determine the number of counter clock periods that it took the photodetector to reach the signal level of that supplied to the comparator input. The device then provides only the number of counter clock periods as an output or signal value associated with the photodetector.
While the disclosure of Konuma does provide increased dynamic range through effectively increasing V.sub.sat, this approach has several problems.
First, if one were to have a counter and comparator in each pixel, the number of components in each pixel is very large leading to a pixel with a very small fill factor or a very large pixel. This approach is not practical given the present minimum feature sizes of state of the art semiconductor technology, and the need for small pixel, low cost image sensors.
Second, the output for each pixel is a counter value for the time required to reach a given threshold, and does not contain an analog output value for the actual amount of charge integrated in the photodetector. With this approach, although the effective V.sub.sat level is increased, the effective DR will be limited by the time period or accuracy of the counter clock, and the size of the counter. For example, if the counter has 10 bits or 1024 counts, the dynamic range is extended to 10 bits provided that the 1024 master clock periods can fit into the desired exposure time. If the desired exposure time were 100 msec., then the counter clock period must be .ltoreq.97.6 usec. If one were try to extend the DR to 20 bits, a 20 bit counter would be required, and a counter clock frequency of &gt;10.5 MHz for an exposure time of 100 msec. As the exposure time decreases a commensurately faster master clock is required. For example, if an exposure time of 1/60.sup.th of a second were desired or required in the case of capturing an image in outdoors in bright sunlight, a master clock of 63 MHz would be required to quantize 20 bits. It is evident that very fast counter clocks are required to provide high dynamic range in typical exposure conditions. Also, as the number of bits in the counter gets larger, more area required to integrate this into the pixel, producing a larger and larger pixel. Typical counters require 4-8 transistors per bit. Thus a 20 bit counter would require 80-160 transistors, yielding pixel sizes of &gt;40 um in a 0.35 um CMOS process. Additionally this approach requires that all pixels within the image sensor reach the programmed threshold level in order to have an output value for each pixel. This would require very long exposure times to allow dark regions of the scene to reach the threshold level if the threshold level is near V.sub.sat. The exposure times could be decreased by programming the threshold level to a very low value, but this would reduce the accuracy of information in very bright regions of the scene since they will reach the threshold value in extremely short time periods.
Thirdly, with the approach of Konuma, at the brightest light levels the data is more quantized. This is shown is FIG. 2. This can be seen by looking at how the effective light measurement is calculated from the time to threshold.
If one knows the amount of time (t.sub.T) required to reach a threshold (V.sub.T) and assume that the source is constant over the time being measured, then one can calculate the amount of light at any arbitrary time, (t.sub.M). The expression for the extended effective voltage (V.sub.ext) is given by Equation 2 below. ##EQU1##
In a discrete system the time variable, t.sub.T, would be measured by a quantized unit as indicated in Equation 3. ##EQU2## Where cv is the quantized integer code value and MaxCv is the code value that corresponds to the cv value at t.sub.M. Substituting values we arrive at Equation 4. ##EQU3##
Referring to FIG. 2, a code value (cv) of zero implies infinite light. The first measurable quantization, which is also the largest, is between cv=1 and cv=2. The quantization for an 8 bit linear system is 0.0039, which is less than the smallest quantization in a time to threshold method described by Konuma.
Fourthly, if one were to have a single counter and comparator used outside of the pixel array to keep track of the time to threshold, each pixel would then have to be measured at an extremely high rate in order to have a small enough sampling frequency per pixel to provide fine enough quantization for extension of the dynamic range. For example, assume that 10 bits of quantization over the desired exposure time is required, and that there are 1 million pixels in the image sensor. Given a desired exposure time of 100 msec., each pixel would have to accessed and measured against the programmed threshold level every 97.65 usec. This means that 1 million pixels need to be sampled every 97.65 usec. This would require a pixel sampling rate of 1 pixel per 97.65 psec, or 10.24 GHz. A means for doing this has not been disclosed by Konuma or elsewhere in the field of APS or other image sensor devices.
Finally, the output value provided is a time. In order to reconstruct the incident image from this output, (i.e. determine the signal level), one must extrapolate by multiplication from the time value. This can degrade the effective noise level of the sensor. The value t is used to measure the time for a voltage v(t) to reach to a threshold. The signal VPD(t) represents the accumulation of photons over time with some Gaussian additive noise with a standard deviation of .sigma..sub.v. One experienced in the art can show that the noise in the extended voltage domain (.sigma..sub.Ext) is related to the additive noise as indicated by Equation 5. ##EQU4##
Given that t.sub.M is always greater than t.sub.T one can see that the value of .sigma..sub.Ext is always greater than .sigma..sub.v. From the foregoing discussion it should be apparent that there remains a need within the prior art for a device that retains provides extended V.sub.sat and dynamic range while retaining low noise, small pixel, simple and low frequency readout, and means to manage the quantization of extended voltage signals.